Much of the water in the environment contains bacteria and other microorganisms that may be harmful to plant and animal life, particularly humans. Since many diseases and illnesses are transmitted to humans through drinking water, it has long been the practice to sterilize water intended for human consumption. In general, this has been accomplished by boiling the water or treating the water with a chemical substance which either itself kills the microorganisms or generates other agents which kill the microorganisms. One of the most common chemical methods for destroying harmful bacteria and other microorganisms in an aqueous stream is to treat the aqueous stream with chlorine or a chemical which generates chlorine upon contact with water. For example, water has been chlorinated by the addition of dry substances, such as sodium hypochlorite, to the water. The sodium hypochlorite decomposes upon contact with water to produce chlorine gas. This method of water sterilization is generally unsuitable for large scale treatment of aqueous streams because of the difficulty of continuously metering the dry sodium hypochlorite into the water.
Another commonly practiced method of sterilizing aqueous streams is to introduce gaseous chlorine into the streams. This method is not always satisfactory for large scale plants since it requires large and expensive equipment to handle and deliver the large quantities of chlorine required for adequate sterilization. For example, chlorine, which is generally provided in liquid form in tanks or cylinders, must be vaporized prior to metering into the aqueous stream. This necessitates the use of large size vaporizers which have considerable electricity or steam requirements and which must be frequently cleaned. Another disadvantage of gaseous chlorine water sterilization is that the rapid high volume draw-off of gaseous chlorine may cause freezing of the chlorine lines due to inadequate heat transfer.
An alternative method of chlorinating aqueous streams which has been practiced involves the introduction of liquefied chlorine into the aqueous streams. The use of liquefied chlorine eliminates many of the difficulties encountered when dry powdered chemical substances or gaseous chlorine are used as the sterilizing agents. This technique is also appealing because liquid chlorine is much more concentrated than gaseous chlorine and therefore smaller equipment can be used. For instance, tubing having a diameter of less than one-half inch is adequate for chlorinating water in a large treatment plant using liquefied chlorine, whereas a pipe having a diameter of several inches is required to provide equivalent mole volumes of gaseous chlorine.
However, as might be expected, the use of liquefied chlorine in the treatment of aqueous streams is not completely problem-free. One of the major problems encountered when liquefied chlorine is used to sterilize aqueous streams is that the aqueous stream being treated sometimes freezes in the vicinity of the point of introduction of the liquefied chlorine. This results from the vaporization of the liquefied chlorine as it leaves the chlorine dispensing equipment and enters the water. The vaporization is caused by the low pressure of the water relative to the pressure in the chlorine dispensing system.
Liquefied chlorine generally is transported to treatment facilities in metal tanks and cylinders. After filling, the liquefied chlorine in the closed container boils until its equilibrium vapor pressure is reached. The equilibrium vapor pressure depends on the ambient temperature, and at normal ambient temperatures it is generally in the range of about 80 to 120 psig. The pressure of the aqueous stream being treated, on the other hand, is generally in the range of about 40-60 psig. Thus, as the liquefied chlorine enters the aqueous stream, it undergoes a significant pressure drop, and if its temperature is at or above its boiling point at the pressure of the aqueous stream, it will vaporize into gaseous chlorine. The heat of vaporization of the chlorine is provided by the water, which itself often undergoes a phase change to ice. If this occurs, the ice may surround the liquefied chlorine injection tube and block further passage of chlorine into the water.
Because of the convenience of using liquefied chlorine for sterilizing aqueous streams, continuing efforts are under way to develop methods of introducing liquefied chlorine into aqueous streams without causing the water surrounding the chlorine injection tube to freeze. For instance, U.S. Pat. No. 3,133,877, issued to Mixer et al., describes a process which purportedly eliminates the difficulties encountered in the earlier methods of introducing liquefied gases into liquids. Mixer et al.'s process comprises raising the pressure of the liquefied gas which is to be introduced into a liquid stream to a pressure which is higher than the pressure of both the source of the liquefied gas and the liquid being treated. Mixer et al. accomplishes this by means of a pressure pump and a back pressure valve. Both are situated between the liquefied gas storage container and the injector tube through which the liquefied gas enters the liquid being treated.
A disadvantage of the process disclosed by Mixer et al. is the considerably high pressure required in the system. The pressure of the liquefied chlorine is maintained in the range of 150-200 PSI. High pressure equipment and fittings must be used because at such high pressures there is a danger of leakage of chlorine to the environment. Furthermore, the process disclosed by Mixer et al., while it may prevent the freezing of chlorine in the pressurized equipment located upstream of the back pressure valve, does not solve the problem of ice formation around the injection tube, which is downstream of the back pressure valve, where the chlorine undergoes a pressure drop and may flash.
U.S. Pat. No. 4,770,198 issued to Bergman described another process for introducing a liquefied gas or mixture of liquefied gases into a liquid. Bergman states that his process prevents the liquefied gas from flashing in the liquefied system by maintaining the pressure of the system above the vapor pressure of the liquefied gas until the liquefied gas is injected into the liquid being treated. Bergman accomplishes this by pressurizing the liquefied gas with an inert gas, such as nitrogen, to ensure that there is no pressure drop and consequent freezing in the liquefied gas system, as a result of a sudden draw off of the liquefied gas. Bergman also provides a specially designed back pressure metering valve which permits the liquefied gas to enter the liquid being treated at the point of pressure drop. Freezing of the liquid being treated can still occur in the Bergman device at the point where the liquefied gas is introduced into the main stream of the liquid because of flashing of the liquefied gas.
The currently practiced methods of introducing a liquefied gas into a liquid stream have not successfully overcome the problem of freezing of the liquid stream at the point of liquefied gas injection; thus, there is an ongoing search for a process which effectively accomplishes this result.